Echanges entre OO et WA

Bonjour!

J'ouvre un sujet relatant les discussions en cours entre OO et WA (à travers moi!).

Je voudrais que ce sujet ne soit pas noyé sous un flot de posts dont l'utilité n'est pas immédiatement liée à cet échange purement informatif; je vous remercie donc de bien vouloir en discuter dans l'autre sujet!

Merci d'avance.

J'ai donc d'abord envoyé ce message à OO, le 2 juin:

Dear John and Barry,

I own OO scopes for many years and I'm very satisfied by your products. This is the reason for which I ask you to acquaint the following facts, it is of first importance for you.

In the forum webastro, actually the most important forum for hobby astronomy in France and Belgium, two members would have sent two of yours mirrors for measurements by the famous optical society REOSC.

The two mirrors are 400mm diameter.

One would have been found at strehl 0,37 and 0,50 for the second, L/1 astigmatism, both would have very rough surface.

The messages are here, #182 and following:

http://www.webastro.net/forum/showthread.php?p=877692#post877692

The control results by REOSC are done in message #186

I appear in the messages as "gégé".

Please, read this messages, I can translate them for you if necessary.

I think that it is of greatest interest for you to realize countermeasures on these mirrors in OO labos, if possible in presence of one of these members Chonum or Thierry Ruiz.

Thierry Ruiz is a manufacturer of high end Dobson in France, Chonum works in opticals and is considered as a good specialist.

Your image is going to be destroyed among french and belgium amators if you do not explain the differences: error in the test procedure and conditions? What else?

I hope very much for you of this message, best regards,

G. SIRVEN

Réponse d'OO, reçue aujourd'hui le 3 juin:

Hello Gerard,

Nice to hear from you, although it would have neen nicer under better circumstances.

First of all, my apologies for writing in English on a French notice board, my French is not good at all I feel that all readers of this board need an explanation as to why there is a problem with some of our mirrors being discussed here. Reluctantly, I am going to have to reveal some facts which may prejudice our legal action against one of our employees who was fired a short time ago. Making this public may not help our case in a prosecution but, I feel it is necessary to give all you group members an explanation. A few weeks ago, we had to dismiss an employee. He worked in our optical finishing department. I will explain: When a mirror is made in our optical departments it is tested by the person who made it, it then gets tested with another technician before it is released to our assembly department. Both these technicians sign the quality document to show who tested it and their results. We found one technician had forged signatures on 6 mirrors, 4 went to France and 2 went to the Far East before we found the problem. This was done to increase his apparent output and visible skills in that department. We have had the two Far East mirrors back and rectified them, one was fine, the other was not. The four in France are still in France and are causing us problems as you all know. We have asked for them to be returned, at our cost, on two separate occasions and they will be rectified also, the customers have not done so. In 99.9% of cases our optics, some of the best in the World, according to our customers, achieve and often surpass our advertised specification. However, of the many thousands of optical surfaces we produce in a year, there is a likelihood the occasional one or two will ‘escape'. But, in 100% of cases where any faulty product is found by a customer, we, without exception, rectify it to the customers 100% satisfaction. Obviously, we can only do this if we have the optics back with us in the UK to check them and, if necessary, replace completely if there is a major fault. If people just hold onto a product which they think, or know is not up to specification, there is no way possible we can help them by replacing the optics. Only when we get them back in the UK can we sort out the problem and ensure our customers are happy. I hope this honest response is accepted in the manner it is offered.

Best Regards Barry Pemberton Managing Director.

A qui j'ai répondu ce même jour:

Hello Barry,

Thank you very much for your answer. I will forward your answer this day on the newsboard.

I also look for what differences in procedure measurement would give such a gap, because I am not sure that the two members of the forum have correctly worked.

I Think:

> The way the mirror is hold during the test (I think the mirror must be hod the same way you did. If you can explain your way, thank you very much, I will forward).

> The temperature must be established before testing. I think at least 4 hours in the labo are necessary before testing.

> What else?

Best regards, I keep you informed.

Réponse d'OO suite à la parution de ses réponses dans le forum (le sujet fermé), elle me semble très importante:

Hi Gerard,

Please feel free to post the following and thanks for your help, you are more than a good customer, you are a good friend.

The method of holding telescope mirrors is unbelievably critical. If they are not supported properly the mirror will develop astigmatism and sometimes, added spherical aberration.

We spent a lot of time experimenting holding mirrors before we developed a method which is 100% efficient. Obviously I will not divulge specific details here but I can say, we use controlled diameter vacuum pads to hold the back of the mirrors. They depend on mirror thicknesses and of course weights.

We never show any sign of astigmatism with this method. If a typical method of holding/supporting a mirror on a wide strap or, on two support points etc. around the edge, you are guaranteed to get astigmatism, especially with large mirrors with a thickness to diameter ratio of less than 1:6. Very easy to add over one wavelength of astigmatism if not held properly.

If a mirror is taken from a warm environment into even a 2 degree cooler environment, it will obviously start to cool. Because the mirror's shape is circular it tends to cool more at the exposed edges of the mirror and progressively less towards the centre. This means the edge area of the mirror 'drops' a little during this cooling and gives the mirror a temporary increase in spherical aberration. Taken to extremes, it is possible in a 500mm mirror to add over 4 wavelengths of spherical aberration this way. This shows how critical it is to make sure the mirror has been in the same temperature for at least 4-6 hours otherwise, any tests are invalid. Similarly, if a mirror is held on a stand for testing in a different temperature, the conductivity of the stand is different to the air around it and uneven cooling can result, producing strange astigmatic/spherical aberration effects.

Another aspect of interferometry testing is the method of gathering the fringes for analysis. If one set of fringes is captured for analysis, any slight air turbulence in the room could distort these and so, any analysis is incorrect. Our Zygo can take up to over 1000 measurements over a few minutes and then total them, and then average to give a very accurate measurement of the wavefront.

This very short descriptive passage is by no means trying to avoid those mirrors in France which seem, at face value, to be wrong but, when/if we get them back, we will again test them and, if they are found to be faulty, we will of course rectify totally free of charge and refund any shipping charges. They are certainly in the group of six we are very suspicious about but only when we get them back here can we be sure and sort any problems out.

Best Regards

Barry Pemberton

Managing Director

Orion Optics

GG